Now imagine him doing all this with the body and resources of an 18-year-old.

You have just pictured both a perfectly normal toddler and a typical violent criminal as Dr. Tremblay, a developmental psychologist at University College Dublin sees them — the toddler as a creature who reflexively uses physical aggression to get what he wants; the criminal as the rare person who has never learned to do otherwise.

In other words, dangerous criminals don’t turn violent. They just stay that way.

“It’s highly reliable,” said Brad J. Bushman, a psychology professor at Ohio State University and an expert on child violence, who noted that toddlers use physical aggression even more than people in violent youth gangs do. “Thank God toddlers don’t carry weapons.”

We start as toddlers. We learn through conditioning, as we heed requests not to hit others but to use our words. We learn self-control. Beginning in our third year, we learn social strategies like bargaining and charm. Perhaps most vital, we use a developing brain to read situations and choose among these learned tactics and strategies.